Flow -> Line Width -> Z offset = confusion

I am just trying to deeply understand relations between those parameters. I am not a noobie, but just trying to be more smart with a printer, instead of changing parameters randomly to see effects on the print.

So Let's Assume Bed is leveled perfectly and all Motor steps are calibrated correctly (X,Y,Z,E)

I do understand some of the correlations e.g.:

1. When Flow is too low, on first layer you have to lower Z-Offset to get nice squish, but next layer will be under extruded, because Z-Offset does not apply on any layer except 1st.
2. When your flow is too high, you have to bring Z-Offset higher, to get first layer perfect, but then next layers will be over extruded and looks squished too much.

Now, I have calibrated flow, with method of printing shallow cube with 2 walls (perimeters). I've set all line widths to be 0.4 (like my nozzle), so on the measurements I expect wall to be 0.8 (2x0.4), some tutorials says I should aim for 0.82, but for now I left it at 0.8 which was pretty much spot on.

Leaving line width's set to 0.4 for all lines and layers, and line height to 0.2 on all layers, I started printing, on the fly I adjusted Z-Offset to get that first layer right. But on every other layer, I had too much squish (over extrusion). And that does not fit the things I though I understand. Because now if I lower flow, flow calibration will come wrong. I also know that by default in most slicers, layer height and line width are not set to same values for all layers.

Of Course I could just fiddle around, keep changing those settings and probably I would finally get what I want, but I want to understand what to change and why, so rather than telling me what I am doing wrong, I would like to know how it should work, I am guessing there is How it should work in ideal world, and how it works with budget printers. But I really really want to understand that part of the printing, and maybe there is other complications I am missing in here.

If the extruder is calibrated, you'll set flow at 100% and it will be fine regardless of line width or first layer thickness settings. If the Z axis is calibrated, when you set the first layer thickness to 0.2 mm, that's what you'll get. If you specify 0.4 mm line width that's what you'll get with flow at 100%.

Why do you want to play with the flow control so much? Extrusion % or flow is exactly what you'd think. Over 100% means more plastic comes out of the nozzle. Less than 100% means less plastic comes out the nozzle. Imagine what that means in terms of a fixed layer height- less plastic means narrower lines, more plastic means wider lines.

You should not have to play with the flow setting all the time. If you do, something is wrong.

The complications you are missing is that the plastic is pressurized in the hot-end and it doesn't behave linearly. There are some firmware fixes for that- look into pressure advance (or whatever the equivalent is in your firmware) and nonlinear extrusion (or whatever it's called in your firmware).

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Well, all this probably comes from reading too many tutorials and articles .

I've came across some articles when they calibrate flow to 0.4 extrusion width (same as nozzle) and expected width of double walls would be 0.8. and that's what I follow.

Also I am about flow, because I've read numerous articles, and everyone pretty much say, that flow is more accurate than E-Steps calibration, and it let's you calibrate it to each filament, in case you need it.

But essentially, it's the same, either calibrating E-Steps or calibrating flow.
In my case I have calibrated E-Steps and also seems like 100% flow seems to produce spot on 0.8 on 0.4 line width (double walls).,

However I still have some thing I don't understand.
With all those calibrations done and confirmed correct, When I am printing something and setting Z-Offset to be okay for first layer. my second and every other layer seems over extruded.
So that's the question here how should I approach that ?

I also printed XYZ cube, and it's spot on in all directions.

E-steps and flow are not independent. You have to fix one in order to calibrate the other. When finished, ignoring nonlinearities, the e-steps value used at 100% flow should give you the specified line width (away from corners of the print).

Two perimeters of 0.4mm should result in a 0.8 mm wide wall, but don't try to use a 20 mm cube for calibration. It's one of those dumb ideas that keeps getting repeated over and over by the 3D printing community, sort of like using 4 leveling screws for the bed.
A 20mm cube is too small to calibrate anything. Pressure advance primarily affects the extrusion as the nozzle goes into and back out of a corner. With a 20 mm cube, it's all corners and they are too close together for calibration purposes. Without tuning pressure advance, corners are typically over extruded and tend to look swollen. Solid infill also gets overextruded where the motion reverses direction (near the edges of the print). And all of this depends on acceleration and jerk settings...

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Yeah I had that argument with someone yesterday when they said I should print 20mm cube and calibrate my axes to that ...

Either way, I do have linear advance calibrated so corners are pretty nice, everything seems pretty nice and spot on.
Like I said I've printed that 0.4 line width shallow cube with 2 walls, and it was bang on 0.8 so flow and E-steps are all good. ( I did calibrated E steps prior that too).

But having all that spot on, I am still having issues with everything after 1st layer. it's definitely over extruded starting from second layer.
I still get prints done, and they look okay.

But also, on some point I've changed line height to be 0.3 not 0.2, and that made it even more visible.

So Again.

I first printed couple of layers with 0.2 height, and adjusted z-offset with baby steps, so first layer was perfect. but then second was slightly over extruded.

But when I moved to 0.3 height of layer, it was even more significant, and even that I set z-offset previously, first layer was way too squished.
What am I missing here?